Biodiesel is a non-petroleum-based fuel made from renewable resources, for example, plants, microorganisms, or the like. It can be used as a diesel fuel substitute or as a component of fuel blends. Biodiesel is biodegradable and non-toxic, so fuel spills pose far less risk to the environment. Further, because the production of biodiesel “recycles” CO2, biodiesel production and use does not significantly contribute to global warming.
Chemically, biodiesel feedstock can be composed primarily of triglycerides (TGs), which can be formed from a single molecule of glycerol with three long chain fatty acids attached.
The figure below shows the transesterification process whereby TGs can be converted into methyl ester biodiesel. To the feedstock can be added an alcohol, for example, methanol as exemplified here, in the presence of a catalyst. The reaction between the TGs and the alcohol is reversible, so the alcohol can be added in excess to drive the reaction and ensure complete conversion.

This transesterification reaction can be catalyzed using an acidic or basic catalyst in order to maintain adequate conversion rates. Though widely used, base catalysis can suffer from limitations that can increase production costs for biodiesel. Acid catalysts can be used, but the use of strong acid catalysts can create a need for corrosion-resistant processing equipment. Additionally, the liquid acid needs to be removed after the reaction is catalyzed.
As an additional constraint, liquid acid and base catalysts can call for the use of relatively “clean” feedstocks. As a result, prior to the transesterification reaction, many biodiesel feedstocks need processing, for example, lysis of microorganisms, crushing of plant matter, removal of water and free fatty acids (FFAs), or the like, or a combination thereof. These pre-transesterification steps can add cost and complexity to the biodiesel production process, and/or shift the net energy balance of production in a negative direction, which can lessen the attractiveness of biodiesel as a fuel.